1. Field of the Invention
The invention relates to an engine control system for controlling combustion of an internal combustion engine equipped with an exhaust gas purifying catalyst, and, more particularly, to a fuel combustion control system for accelerating a rise in temperature of an exhaust gas purifying catalyst installed in an exhaust line during a cold start of the engine.
2. Description of the Related Art
Typically, there have been known various types of catalytic converters for purifying exhaust gas from an engine. Such a catalytic converter incorporates a three way catalyst to purify or significantly lower emission levels of unburnt hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx) and the like which can pose a health problem for the nation if uncontrolled. The three way catalyst is hard to present desired catalytic conversion efficiency if it is at lower temperatures and is, however, activated to present catalytic conversion efficiency when heated higher than a specific temperature sufficiently.
In this type of engine control system, it has been known to accelerate a rise in catalyst temperature by greatly retarding an ignition timing after top dead center while the catalyst has not yet attained an activated condition necessary to present desired catalytic conversion efficiency which is dictated based on a temperature of engine cooling water. One of the engine control systems of this type is known from, for example, Japanese Unexamined Patent Publication No. 8-232645. The engine control system causes a large retard of an ignition timing to provide a large amount of exhaust heat loss with an effect of rising exhaust gas temperature, as a result of which, the catalyst is quickly heated and suitably activated. Further, in order to prevent fuel combustion from being made unstable due to retarded fuel ignition, the prior engine control system is designed and adapted to improve ignitability and combustibility of an air-fuel mixture by generating a swirl of intake air in a combustion chamber, increasing a speed of an intake air stream or rising ignition energy.
It has been known as an effective control for rising exhaust gas temperature to shift an air-fuel ratio to a lean side as well as to retard an ignition timing. However, there is a somewhat conflicting problem between these control that a rise in exhaust gas temperature and stabilization of combustion are contrary to each other. Specifically, retarding an ignition timing and shifting an air-fuel ratio to a lean side in order to rise exhaust gas temperature are apt to make combustion unstable. In particular, since fuels practically used differ in quality such as the degree of fuel heaviness from one another, there is a fear of unstable combustion beyond an allowable limit due to variations of ignitability and combustibility of the fuels, which always causes engine vibrations and a sharp increase of harmful emissions. Because, while the engine is not yet warmed up, the catalyst remains inactive, there is a demand for controlling the level of harmful emissions as low as possible.